An ultrasound transducer is typically fabricated as a stack of multiple layers that depend on the application of the transducer. FIGS. 1a and 1b show typical ultrasound transducers. Each transducer comprises, from the bottom up, a backing layer 30, a bottom electrode layer 17, an active element layer (e.g., piezoelectric element or PZT) 10, a top electrode layer 13, a matching layer (or multiple matching layers) 20, and a lens layer (for focused transducers) 35 and 45. The lens may be a convex lens 35 or a concave lens 45. The backing, matching and lens layers are all passive materials that are used to improve and optimize the performance of the transducer. The backing layer is used to attenuate ultrasound energy propagating from the bottom of the transducer so that ultrasound emissions are directed from the top of the transducer and the matching layer is used to enhance acoustic coupling between the transducer and surrounding environment. Different transducer designs (different sizes, frequencies, applications, etc.) require passive materials with different acoustic properties. Therefore, there is a need for effective methods to control the acoustic properties of these materials to deliver consistent performance while maintaining manufacturability and compliance with processing methods.
A common method to control the properties of passive layers is to add different fillers in different quantities to an epoxy or polymer to create a matrix. Common filler materials include tungsten, alumina, and silver (e.g., in powder form). For example, silver is used in very high quantities to make an otherwise insulating epoxy conductive. Tungsten and alumina are used to control the acoustic impedance of the passive layer by varying the filler/epoxy matrix density. Although the method of using fillers has several advantages in terms of flexibility, simplicity and cost, it also has several drawbacks. This method can only raise the acoustic impedance up to a certain point after which the epoxy saturates and will not mix with any additional filler. Also, the filler can move around in the epoxy before the epoxy is cured, making it difficult to control the final distribution of the filler in the epoxy. Another drawback with tungsten and alumina is that the composite material remains nonconductive. Another drawback is that changing the composition of the passive layers in many cases also affects their manufacturability.
Some of these drawbacks can be overcome by adding more processing steps or using novel mixing, casting and fabrication techniques. However, these techniques eliminate the main advantage of using filer/epoxy matrices, which is simplicity and flexibility.
Therefore, there is a need for passive layers and fabrication methods that provide high flexibility and manufacturability without sacrificing performance or cost.